The dirty little secret of flash drives today is that many of them are running on yesterday's interfaces. While SATA and SAS have undergone several iterations since they were first introduced, they are still based on decades-old concepts and were initially designed with rotating disks in mind. These legacy protocols are bottlenecking the potential speeds possible from today's SSDs.
Slow computer got you down? Maybe a hardware upgrade should be in your cards, if so this guide to the Fundamentals of Upgrading a PC is an invaluable read. PCSTATS Tips